(Adds Mashable report on documents)

By Lisa Richwine and Jim Finkle

LOS ANGELES/BOSTON, Dec 8 (Reuters) - A group that claimed to be responsible for the massive computer hack at Sony Pictures Entertainment demanded the company cancel the release of "The Interview," a film comedy that depicts an assassination plot against North Korea's leader.

A letter posted on a file-sharing site on Monday asked Sony to "stop immediately showing the movie of terrorism which can break the regional peace and cause the War!" It was signed by GOP, the nickname for the "Guardians of Peace" group that says it is responsible for a cyber attack at Sony that began Nov. 24.

Pyongyang has denounced "The Interview" as "undisguised sponsoring of terrorism, as well as an act of war" in a letter to U.N. Secretary-General Ban Ki-moon.

People close to the investigation of the Sony hacking have told Reuters that North Korea is a principal suspect, but a North Korean diplomat has denied that his nation is involved.

The letter included links to downloads of several gigabytes of new data purported to have been stolen from Sony. Reuters was not able to verify whether the letter or documents were released by the same group that revealed other Sony documents.

The letter also said the GOP was not involved in a threatening e-mail sent to Sony staff on Friday. That e-mail claimed to be from the group.

The documents released on Monday included an e-mail to Sony that demanded "monetary compensation" to avoid "great damage" to the studio, according to the website Mashable. The e-mail was dated Nov. 21, Mashable said in its report. News of the hacking became public on Nov. 24.

A Sony spokesman had no comment on the new letter or the Mashable report. Sony Pictures Entertainment is a unit of Japan's Sony Corp.

"The Interview," starring James Franco and Seth Rogen, is scheduled for release in the United States and Canada on Dec. 25. The studio is holding advance screenings for media and others.

(Reporting by Lisa Richwine and Jim Finkle; Editing by Dan Grebler and Ken Wills)